Fighting turrets is so frustrating..

The grenade arc keep snapping onto the turret. I want to throw it around and hit the enemy mounting it. How do I make the auto-targeting thingy stop!? Or at least be able to make it snap onto the enemy and not the turret. (thought I think that would kind of take away from the challenge..)

In previous levels I was able to bounce a grenade off a wall or something so that it hits the guy not the turret, which is the only way I can do so without snapping onto the turret. But I'm in chapter 3-1 and encountered a turret in which I was not able to do this. It's freakin cheap. Help appreciated.